1. Field of the Invention
This invention relates generally to the fabrication of a giant magnetoresistive (GMR) magnetic read head, more specifically to the use of novel seed layers in the fabrication of single and double spin valve sensors for reading ultra-high density recorded media.
2. Description of the Related Art
Early forms of magnetic read heads decoded magnetically stored data on media such as disks and tapes by making use of the anisotropic magnetoresistive effect (AMR) in magnetic materials such as permalloy. This effect was the change in the electrical resistance, r, of certain magnetic materials in proportion to the angle between the direction of their magnetization and the direction of the current flow through them. Since changing magnetic fields of moving magnetized media, such as magnetically encoded tapes and disks, will change the direction of the magnetization in a read head, the resistance variations of the AMR effect allows the information on such encoded media to be sensed and interpreted by appropriate circuitry.
One shortcoming of the AMR effect was the fact that it produced a maximum fractional resistance change, DR/R (where DR is the change in resistance between the magnetic material subjected to its anisotropy field, Hk, and the material subjected to zero field), which was only on the order of a few percent. This made the sensing process difficult to achieve with accuracy.
In the late 1980's and early 1990's the phenomenon of giant magnetoresistance (GMR) was discovered and soon applied to read head technology. The GMR effect derives from the fact that thin (≅20 angstroms) layers of ferromagnetic materials, when separated by even thinner (≅10 angstroms) layers of conductive but non-magnetic materials, will form ferromagnetic (parallel spin direction of the layers) or antiferromagnetic states (antiparallel spin direction of the layers) by means of exchange interactions between the spins. As a result of spin dependent electron scattering as electrons crossed the layers, the magnetoresistance of such layered structures was found to be significantly higher in the antiferromagnetic state than the ferromagnetic state and the fractional change in resistance was much higher than that found in the AMR of individual magnetic layers.
Shortly thereafter a version of the GMR effect called spin valve magnetoresistance (SVMR) was discovered and implemented. In the SVMR version of GMR, two ferromagnetic layers such as CoFe or NiFe are separated by a thin layer of electrically conducting but non-magnetic material such as Cu. One of the layers has its magnetization direction fixed in space or “pinned,” by exchange anisotropy from an antiferromagnetic layer directly deposited upon it. The remaining ferromagnetic layer, the unpinned or free layer, can respond to small variations in external magnetic fields such as are produced by moving magnetic media, (which do not affect the magnetization direction of the pinned layer), by rotating its magnetization direction. This rotation of one magnetization relative to the other then produces changes in the magnetoresistance of the three layer structure.
The spin valve structure has now become the implementation of choice in the fabrication of magnetic read head assemblies. Different configurations of the spin valve have evolved, including the bottom spin valve, wherein the pinned layer is at the bottom of the configuration and the top spin valve, wherein the pinned layer is at the top. In addition, the qualities of the spin valve have been improved by forming the pinned layer into a synthetic antiferromagnet, which is a layered configuration comprising two ferromagnetic layers separated by a non-magnetic coupling layer, wherein the ferromagnetic layers are magnetized in antiparallel directions. The present challenge to the spin valve form of sensor is to make it suitable for reading recorded magnetic media with recorded densities exceeding 20 Gb/in2. This challenge can be met by making the free layer extremely thin, for improved resolution in the track direction, while not reducing Dr/r, which is a measure of the sensor's sensitivity. One way of achieving this goal is by forming the spin valve on a seed layer, which is a layer of material whose purpose is to improve the crystalline structure of magnetic layers grown upon it. Many spin valves are formed on seed layers of Ta. Huai et al. (U.S. Pat. No. 6,175,476) disclose a synthetic spin valve sensor in which the seed layer can be Ta in a high resistivity phase. The present inventors have already shown that spin valves fabricated using a NiFeCr seed layer have a greatly enhanced GMR effect as measured by Dr/r. Presently, the NiFeCr seed layer is becoming the industry standard for heads capable of reading densities exceeding 20 Gb/in2. The Cr composition of the seed layer in these heads is between 20 and 50 atomic percent, with the optimum value of Dr/r obtained with 40 atomic percent. Lee et al. (U.S. Pat. No. 6,141,191) disclose a top spin valve using a NiFeCr seed layer wherein the atomic percentage of Cr is between 20 and 50%. Lee et al. note a Dr/r for the configuration of 7.7%. Huai et al. (U.S. Pat. No. 6,222,707) disclose single and dual bottom spin valves using NiFeCr seed layers with a range of Cr atom percent between 20% and 50%, with approximately 25 atomic percent being preferred. They note an improvement in the texture of a synthetic antiferromagnetic pinned layer. The head so formed was characterized by DR/R between 9.42 and 10.19 and by sheet resistances between 12.77 and 14.9 . Subsequently, it was shown that a synthetic pinned layer spin valve head made with a NiCr seed layer having 40 atomic percent of Cr yielded better synthetic pinned layer properties than such a head made with a NiFeCr seed layer having 40 atomic percent of Cr. In a very early patent, Lee et al. (U.S. Pat. No. 5,731,936) provide such NiCr and NiFeCr seed layers in read heads using permalloy magnetoresistive elements. Aoshima et al. (U.S. Pat. No. 6,046,892) disclose a bottom spin valve read head with CoFeB free and pinned layers and using a Ta/NiFeCr seed layer wherein the Cr is at 24.3 atomic percent. Fukagawa et al. (U.S. Pat. No. 6,322,911) teach the formation of a seed layer to enhance the (111) crystalline orientation of subsequently deposited antiferromagnetic or ferromagnetic layer. Fukugawa teaches an NiCr or NiFeCr alloy with the addition of an element X of atomic percent between 0.1 and 15%, that will bond to oxygen more readily than the Cr.
A top spin valve read head with NiCr and NiFeCr seed layers has been commercially used for reading recorded densities between 10 and 20 Gb/in2. This head has a free layer of NiFe(60 A)/CoFe(10 A) and a read head width of approximately 0.3 microns. For reading recorded densities of about 30 Gb/in2, a NiCr seed layer with 40 atomic percent Cr still suffices when a NiFe(40 A)/CoFe(10 A) free layer is used together with a read head width of approximately 0.24 microns. For reading recorded densities of about 45 Gb/in2, a NiCr seed layer with 40 atomic percent Cr is incorporated into a bottom spin valve configuration with a CoFeB(10 A)/NiFe(20 A) free layer and a read head width of approximately 0.17 microns. The configuration of this read head provides marginal performance in terms of signal-to-noise ratios and other benchmark parameters. To achieve capabilities of reading area densities exceeding 45 Gb/in2, a magnetic read width of 0.15 microns or smaller is indicated along with an ultra-thin free layer of the form CoFeB(5 A)/NiFe(20 A). In addition, a thinner antiferromagnetic pinning layer of MnPt would be needed to reduce current shunting. A bottom spin valve configuration (see below) meeting these requirements has been produced with the following specific materials and thicknesses, yet its performance would be inadequate for reading area densities of 60 Gb/in2:NiCr(40%)60/MnPt100/CoFe15/Ru7.5/CoFe20/Cu18/OSL/CoFeB5/NiFe20/Ru10/Ta10.In the above configuration the numbers (other than the 40%) refer to approximate thicknesses in angstroms. The NiCr seed layer has 40% atom percent Cr. MnPt is the antiferromagnetic pinning layer, CoFe/Ru/CoFe is the synthetic antiferromagnetic pinned layer, Cu is the spacer layer, OSL represents an oxygen surfactant layer formed on the Cu spacer layer, the surfactant layer being a sub-monolayer of oxygen deposited on the Cu surface by exposing the Cu layer to low-pressure oxygen in a separate chamber, CoFeB/NiFe is a composite free layer formed on the surfactant layer and Ru/Ta is a composite capping layer. The configuration provides a DR/R of 12.7% and a sheet resistance of 19.6
Clearly, to achieve acceptable performance for reading 60 Gb/in2, a novel improvement of the configuration above will be required. It is the purpose of this invention to provide that improvement by the simple means of changing the seed layer from NiCr(40%) to NiCr(31%), which will also allow a thinning of the synthetic pinned layer. The head so formed will have a thinner free layer than any of those disclosed in the prior art cited above. Due to a significant reduction in the thickness of the antiferromagnetic pinning layer, the head will have a thinner overall profile. It will also have values of DR/R and sheet resistivity that indicate a higher sensitivity than the prior art heads cited above. The NiCr(31%) seed layer can also be incorporated into a symmetric dual spin valve configuration, in which a free layer is centered between synthetic antiferromagnetic pinned layers that are positioned above and below it. The novel seed layer allows such a dual configuration to be formed with sufficient overall thinness and sensitivity to meet the requirements of reading storage media with area densities exceeding 60 Gb/in2.